Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples of device-to-device communication include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum.
Device-to-device communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource. Moreover, (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signaling directly between one another. Control signaling transmitted directly between devices (i.e., as device-to-device communication) is referred to herein as direct control signaling. One example of such direct control signaling is the so-called discovery signal (also known as a beacon signal). A discovery signal at least carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. Other devices can scan for the discovery signal. Once the other devices have detected the discovery signal, they can take the appropriate action. For example, the other devices can try to initiate a connection setup with the device transmitting the discovery signal.
When multiple devices transmit direct control signaling (discovery signals or any other type of direct control signaling), the transmissions from the different devices may be time synchronized (mutually time-aligned) or unsynchronized. Synchronization could be obtained for example by receiving appropriate signals from an overlaid cellular network, or from a global navigation satellite system such as a global positioning system (GPS). Discovery signals transmitted by a device within a cell for instance are typically synchronized to a cell-specific reference signal transmitted by the cell. Even in unsynchronized deployments, it may be beneficial for different cells to synchronize to each other, maintaining a time resolution up to that obtainable from the backhaul. If the network time protocol (NTP) is the source of synchronization, typical synchronization drifts are in the order of +/−5 ms.
Unsynchronization could occur where discovery signals are transmitted between unsynchronized cells, carriers and/or public land mobile networks (PLMNs). According to ProSe requirements, wireless communication devices belonging to one cell need to be able to discover wireless communication devices camping on another cell. Additionally, the proximity wireless communication devices may camp on different PLMNs or different carriers. Where different cells, carriers, or PLMNs are unsynchronized, from a device-to-device communication perspective, there are no cell boundaries.
The ProSe Study Item recommends supporting device-to-device communication for out-of-network coverage wireless communication devices. In such case, different synchronization options are possible: wireless communication devices may synchronize to a global reference (e.g., GPS) which is in general different from the synchronization reference of deployed networks. Alternatively, wireless communication devices may operate in a fully asynchronous fashion (no synchronization reference). A further option is that clusters of wireless communication devices synchronize to a specific wireless communication device, such as a Cluster Head (CH). This CH provides local synchronization to its neighbor wireless communication devices. Different clusters are not necessarily synchronized.
Wireless communication devices may discover unsynchronized discovery signals on a given carrier (or subband) by searching for discovery signals in time over their configured/predefined resources. This can be done, e.g., by time domain correlation of the received signal with the discovery signal waveforms. This is similar to the way wireless communication devices search for cells in a long-term evolution (LTE) standard for wireless communication. LTE uses a primary synchronization signal (PSS) and a secondary synchronization signal (SSS).
Wireless communication devices may alternate between an awake state and a sleep state (i.e., discontinuous reception (DRX)). During a sleep state, the memory and clocks are active, but the wireless communication device is configured to not monitor for any direct control signaling. During an awake state (or wake up time), the device is configured to indeed monitor for direct control signaling. Not monitoring for direct control signaling during the sleep state reduces the device's power consumption.